Patent GB-1398184-A (GB)
Title: Sensing apparatus
IPC: G01R19/02
Filed: 1972-06-16
Abstract: 1398184 Measuring rms and mean values of current waveforms WESTINGHOUSE ELECTRIC CORP 16 June 1972 [23 July 1971] 28243/72 Heading G1U The rms value of a current from a current transformer 1, Fig. 1, is determined by rectifying 6 the current, causing it to flow through a load 16 comprising two transistors 12, 14 connected as diodes, using the resulting voltage across the load - which will vary substantially logarithmically with the current - to control, by means of a transistor 24, the current in a second circuit 22, which also includes a transistor 26 connected as a diode, shunting the alternating component of the current in the second circuit away from the diode connected transistor 26 by a capacitor 28 connected either across the transistor 24, in which case the A.C. component circulates around transistor 24 and capacitor 28, or across the transistor 26 as in Fig. 2, (not shown) in which case the A.C. component flows through a loop including transistor 24 and capacitor 28, and measuring 21 the resulting direct current flowing in the second circuit. A modified circuit Fig. 3, (not shown) includes a switch whereby either the rms or the mean value of the unknown current can be determined, the action of the switch being effectively to connect capacitor 28 across the meter 21, thereby to shunting the A.C. component of the current in the second circuit around the meter but allowing it to pass through the diode connected transistor 26. An approximate analysis of the circuit shown in Fig. 1 is as follows. Let i x  be the instantaneous value of the rectified unknown current; let i = i o  + i ac  be the instantaneous value of the current in the second circuit, where i o  is the D.C. component and i ac  is the A.C. component, and assume that the A.C. component flows entirely around the loop consisting of transistor 24, capacitor 28. Then the voltage across the first circuit, ie between conductors 8 and 10, can be derived in two ways: firstly as the voltage across the load 16, which is - log k 1 i x  + log k 2 i x  where k 1  and k 2  are constants associated with transistors 12 and 14; and secondly as the emitterbase voltage plus the diode voltage in the second circuit, which islog k 3 i + log k 4 i o  where k 3  and k 4  are constants associated with transistors 24 and 26. In practice all the above k's may be assumed to be substantially unity, so thatlog i x  + log i x.  = log i + log i o  ie, i x <SP>2</SP> = ii o . But i = i o  + i ac,  and 1/T#<SP>T</SP> O ix<SP>2</SP> dt =1/T#<SP>T</SP> O i o <SP>2</SP> dt + 1/T#<SP>T</SP> O i o i ac  dt hence, =i o <SP>2</SP> providing that T is the period of the alternating component i ac  and is substantially constant over such period. Thus the D.C. component i o  of the current in the second circuit 22 is a measure of the rms value of the current in the first circuit 16. When capacitor 28 is connected across the diode connected transistor (26), Fig. 2 (not shown) the A.C. component also flows through the meter 21 which must therefore be sufficiently damped to indicate only the D.C. component. The current whose rms value is determined could, of course, be derived from a voltage source Fig. 2 (not shown).